Sound recording apparatus



Nov. 30, 1943. H. I. REISKIND 2,335,612

SOUND RECORDING APPARATUS Filed Oct. 19, 1940 2 Sheets-Sheet l imvcntor fil'Zlol 152197201023 5 Manley Nov. 30, 1943. H. 1. REISKIND SOUND RECORDING APPARATUS 2 Sheets-Sheet 2 Filed Oct. 19, 1940 Imventor Gttorneg Patented Nov. 30, 1943 SOUND RECORDING APPARATUS Hillel I. Reiskind, Camden, N.

1, assignor to Radio Corporation of America, a corporation of Delaware Application October 19, 1940, Serial No. 361,835

Claims.

This invention relates to an improvement in round noise reduction circuits for use in film sound recording and, more particularly, to an improvement in the timing of such circuits which permits-them to more accurately follow the envelope of the sound waves.

Ground noise reduction circuits heretofore used of the general type known as peak-reading circuits include a rectifier for rectifying one or both of the halves of the electrical impulses corresponding with the sounds being recorded and an appropriate filter circuit for smoothing out the impulses into a direct current more or less corresponding to the envelope of the sound waves. Such circuits have heretofore been deficient in their performance in a number of respects. One of the diificulties of the performance has been that, the moment a sound peak has passed, the potential in the circuit would start to fall exponentially and would continue this fall until zero potential was reached or until another peak came along whose voltage was greater than the voltage to which the circuit had discharged. In the case of complex waves, this often resulted in the clipping of the peaks of successive waves unless an excessive amount of margin or clearance over the peaks of the waves was provided. In the case of low frequencies, the ground noise reduction shutter or equivalent device also tended to follow the wave shape to a certain extent, thereby decreasing the amplitude of reproduction of those frequencies, and, in the case of full wave rectification, introducing second harmonics.

In order to produce the most desirable results, the ground noise reduction amplifier should, after charging to a voltage determined by the amplitude of the audio wave, retain that charge or Voltage for a predetermined period of time and should then fall off approximately exponentially. If a circuit accomplishes this result, the overall time of discharge can be made substantially the same as in the case of the exponential discharge and a reduction in ripple will be obtained.

When the input signal consists of a series of impulses, the voltage produced by the delayed discharged circuit will more nearly approximate the voltage produced by a sine wave signal of the same peak amplitude than in the case of the normal exponential discharge.

Conversely, a faster closing time may be used withoutincreasing the ripple amplitude or decreasing the peak-reading ability.

I accomplish these results by separately rectifying two impulses corresponding to the same half of the sound wave and then feeding the rectified impulses in opposition into two series circuits having different time constants. The voltage of the rectified impulses may be different or may be rendered different by an amplifier and the time constants of the two circuits may be variously adjusted according to the results to be accomplished. I may even delay the action of one of the circuits until a predetermined point has been reached in the operation of the other circuit and thereby still further modify the resulting characteristlc curve of the apparatus.

One object of the invention is to provide an improved ground noise reduction amplifier,

Another object of the invention is to provide a ground noise reduction amplifier which will have an improved discharge characteristic.

Another object of the invention is to provide an improved ground noise reduction amplifier which, on the cessation of input, will maintain a substantially constant voltage for a brief period and will thereafter cause a decrease in voltage.

Another object of the invention is to provide a ground noise reduction amplifier in which the peak-reading characteristics may be made more nearly independent of the final discharge rate.

Other and incidental objects will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings, in which Figure 1 is an illustration of the rectifier, filter and output tube of one present type of ground noise reduction amplifier which is used as the basis for the improvements shown in the other figures of the drawings,

Figure 2 shows an idealized discharge characteristic curve for a ground noise reduction amplifier,

Figure 3 shows one form of my improvement on the circuit of Fig. 1 in which two opposing potentials are used in two circuits of difierent discharge times,

Figure 4 shows a modification of the circuit of Fig. 3, in which the charging time of one of the circuits is altered,

' Figure 5 is a characteristic curve showing the approximate attainment of the idealized curve of Fig. 2 by the circuit shown in Fig. 1,

Figure 6 shows the characteristic charging curves of the circuits of Fig. 3 and Fig. 4,

Figure 7 show a modification of the invention for eliminating the effect of the cathode heater leakage in the rectifier circuit,

Figure 8 shows a further form of the invention which has the advantages of the form shown in Fig. 7 and which, at the-same time, eliminates some of the disadvantages of the previous forms of the invention,

Figure 9 shows a further modification of the invention which eliminates the additional amplifier tube of Figs. '7 and 8 and which, at the same time, presents substantially the advantages of those forms of the invention, and

Figure 10 is a characteristic curve showing a disadvantage of the form of the invention shown in Figs. 3, 4 and 7, which is substantially eliminated in the forms shown in Figs. 8 and 9.

In the circuit shown in Fig. 1, the audio frequency input is fed through the transformer I to the rectifier II. The rectified current passes to the capacitor l2 which it charges and it passes through the resistor M to the capacitor l which is also charged. The potential across the capacitor i5 is applied to the grid of amplifier tube It, the output of which controls the ground noise reduction device either directly or indirectly. When the input through the rectifier ll ceases, the charge on the capacitor l2 leaks off through the resistor l3 and the charge on the capacitor [5 leaks off through the resistors 13 and M. Since the values of the resistors l3 and id are constant, the potential applied to the grid of the tube l6 falls exponentially and the output of that tube is changed correspondingly. This circuit has the disadvantage that the discharge of capacitor l5 and the fall of grid potential on the tube l6 starts immediately on the cessation of signal and, even if a signal is merely decreased in amplitude, at corresponding fall of potential occurs. If a complex wave of low frequency is fed into the circuit of Fig. 1, there may be a tendency to clip the low frequency peaks due to the immediate starting of the discharge and,

, correspondingly, if a higher frequency varying in amplitude at a lower frequency is applied, there may be a tendency to clip the amplitude peaks,

unless the circuit is so adjusted as to provide excessive margin.

These difficulties would be avoided if the beginning of the discharge of the capacitor I5 or the beginning of the reduction of potential on the grid of the tube l6 could be delayed for a brief period, as shown in the curve of Fig. 2, where the desired grid potential is plotted against time. Although the curve of Fig. 2 is an idealized curve and is not practically attainable at the present time, it may be closely approximated by the circuits hereinafter described.

In Fig. 3, a transformer 20 with two secondaries is substituted for transformer l0 and the secondary 2| thereof is connected in the same manner as is the secondary of the transformer H] of Fig. l. The rectifier II is indicated as one-half of a double rectifier, which may be one of the type ordinarily used for full wave rectification, or it may be a single vacuum tube, or other appropriate rectifier. The capacitors l2 and I5 and the resistors l3 and I4 function in the same manner as described above in connection with Fig. l. The winding 23 of the transformer 20 has a rectifier 22 connected thereto. This winding and rectifier is so connected that it rectifies the same half of the audio frequency wave that is rectified by the rectifier H, but with the polarity reversed Where the circuits are connected. The rectified current from the rectifier 22 is applied across the resistor 24 to the capacitor 25. As shown in Fig. 3, the resistor 24 is in series with the resistor I3 and the capacitor is in series with the capacitor 12, so that the potential E devoloped across the capacitors l2 and 25 in series is fed through the resistor M and applied to the capacitor l5 and the grid ofthe tube Hi. If all the characteristics of the two halves of the rectifier circuit were identical, obviously the voltages would be the same, the discharge characteristics would bethe same, and no potential would be applied to the grid of the tube l6. However, the winding 2! of the transformer is designed to give a higher voltage than the winding 23 and the resistor 24 and capacitor 25 are given such values in relation to the resistor l3 and the capacitor l2 that the time constant of this opposing half of the circuit is much faster than that of the main or controlling half of the circuit. If E1 represents the potential developed across the resistor l3; and E2 represents the opposing potential developed across th resistor 24, the potential E applied to the grid of the tube It is equal to the difference. If E1 is made equal to kill, then E2 will equal (lo-DE, and the combined discharge time characteristic will be -t TBEFn 1 7256.

By proper choice of the constants of the circuit, the shape of the discharge wave may be controlled.

The combined result is illustrated in Fig. 5, which shows the voltage E as the difference between E1 and E2, and remaining substantially constant for a brief period of time, then falling very slowly as the rate of fall of E2 decreases, and then falling more rapidly along the normal slope of the fall of the voltage E.

By proper choice of constants, the charge time of the two circuits can be made the same.

Figure 4 shows a modification of the circuit of Fig. 3 in which the capacitor 25 is prevented from charging so rapidly by the insertion of the resistance 30. If desired, an additional capacitor 3| may also be added. The reason for this circuit is apparent from an inspection of Fig. 10, which shows the type of voltage E developed by the circuit of Fig. 3 when a sine wave signal is applied. It will be seen that, during the negative half cycle of the sine wave, both the condensers l2 and 25 are discharging through the resistors l3 and M respectively. Since the time constant 24, 25 is less than l2, l3, condenser 25 will have lost a greater percentage of its original voltage than condenser l2. As a result, at the next positive half cycle, the condenser 25 will start charging earlier in the cycle than condenser l2, producing the sharp reduction in the resultant voltage shown in Fig. 10. This will not occur if the sine wave is of low enough frequency so that the condenser I2 is completely discharged during the negative half cycle. With the circuit of Fig. 4, this effect is materially decreased, since the resistor 30 delays the charging of the capacitor 25. The values indicated on this figure are values which have been found satisfactory in practice for this type of circuit. It will be understood that Fig. 4 shows only a portion of the circuit-corresponding to Fig. 3 and that the resistor l4, capacitor l5, and amplifier it are connected to the right-hand side of th circuit, as shown in Fig. 3.

The difference in the charging characteristic of Figs. 3 and 4 is shown in Fig. 6. Here E3 shows an exponential charge and represents the voltage E which would be obtained in Fig. 3 during the charging part of the cycle with the constants chosen so that circuits including condensers l2 and 25 have the same charging time. In the case of Fig. '4, the circuit including condenser 25 has a longer charging time than the one which includes condenser I2. The charging characteristic of the combination producing the voltage E is shown as E4. Here the charge starts at a more rapid rate.

In the forms of the invention shown in Figures 3 and 4, it will be apparent that leakage between heaters and cathode may act as a shunt across R--24.

Any possibility of heater leakage is avoided in the circuit shown in Fig. 7. In this circuit, the parts which are similar to those of the other figures carry similar reference numerals, but it will be apparent that, since the rectifier 22 is inserted in the opposite leg of the secondary 23, the voltages across resistors I3 and 24 are in phase. The rectified current from this rectifier produces a potential across the resistor 24 which is applied to the grid of tube 40, which, in turn, produces a reversed potential drop across the tube 40. The amount of this potential drop is determined in the usual fashion by proper choice of tube and resistor value to provide the appropriate voltage E2. A battery or other source of potential 42 may be provided to balance the plate voltage applied to the tube 40 and thereby prevent the application of such external voltage to the lower end of the condenser I5. In this arrangement, the voltage E is developed across the lines indicated and applied across the resistor I4 and capacitor I5 in the same manner as in the other circuits.

The form of the invention shown in Fig. 8 even more effectively obviates the tendency toward a characteristic such as shown in Fig. 10. In this form of the invention, the transformer I0, rectifier II, and filter I2, I3 operate as in Fig. 1, but the control voltage from which opposing voltage is derived is taken from across the terminals of the resistor I3. This voltage is applied across the cathode and grid of the tube which is provided withthe usual cathode resistor 5| and by-pass condenser 52 and the resilient voltage developing across the tube 50 is applied through the capacitor 54 to the transformer 55. The transformer 55 and rectifier 56 are so connected as to pass onlysignals produced by an increase of potential across condenser I2. The output of the transformer 55 is rectified by the rectifier 56 and applied to the timing circuit including the resistor 24 and capacitor 25, as shown. The resulting potential is applied through the resistor I4 and across the capacitor I5 to the tube I6, as in the preceding circuits. It will be apparent that, in this arrangement, any signal applied to the grid of the tube 50 is determined by the potential built up across the resistorI3 and the condenser I2 and, therefore, the

output from the rectifier 55 cannot precede an increase of potential across condenser I2 from the rectifier H, and the sharp dip in the charac teristlc curve shown in Fig. 10 is completely avoided.

A high frequency booster consisting of the reactance 5'I shunted by the resistor 58 is inserted in the plate circuit of the tube 50 in order to compensate for the frequency characteristic of the timing circuit I2, I 3.

In the form of the invention shown in Fig. 9, approximately the same result is accomplished as in Fig. 8, but in an entirely difierent manner. In

this form of the invention, the output of the" the preceding figures. However, a voltagedividing and transformer-loading network including the resistor 62, the resistor BI, and the rectifier 50 is placed across the secondary of the transformer I 0 to supply current to the rectifier 22. In this network, the resistors BI and 62 are so proportioned as to supply to the rectifier 22 the proper portion of the voltage applied to the rectifier II. The rectifier 60, however, is preferably of the copper oxide type, as described and claimed in Dimmick application Serial No. 258,813, filed September 28, 1940, entitled Film sound recorders, and assigned to Radio Corporation of America (RCV D-7554). Due to the fact that the rectifier 60 will pass no current below a certain voltage, which voltage depends upon the number of sections of rectifier used, current will pass through the thermionic rectifier I I producing a potential across the capacitor I2 and resistor I3, and at least a portion of this will be applied through the resistor I4 to the grid of the tube I5 before any appreciable signal is passed through the rectifier 22. During this portion of the operation, the only load on the transformer I0 is that due to the resistor I3, and the voltage E will therefore be higher than when the transformer is loaded through the rectifier 60. The inverse voltage timing circuit, in this case as in the other cases, is composed of the resistor 24 and the capacitor 25. It will be apparent that, independent of the speed of this timing circuit, appropriate choice of the number of rectifier elements in the rectifier 60 will determine the minimum voltage impressed on the rectifier I I before any signal reaches the rectifier 22, and the sudden dip in the characteristic curve indicated in Fig. 10 may be completely avoided.

It will be apparent to those skilled in the art that the values to be chosen for the voltages, resistances and capacitors must be determined in accordance with the use which is to be made of the circuit and the apparatus with which it is intended to cooperate. It will also be apparent that the values need not remain fixed but may be adjustable, so that they may be varied from time to time according to the varied requirements of the circuit. For example, the resistors I3 and 24 in each of the circuits may be made adjustable and may be interconnected so that, if the resistor I3 is adjusted to change the timing of the circuit, the relation of the timing of the two portions of the circuit remains the same and the shape of the characteristic curve remains similar except for its expansion or compression along the time axis. A set of typical values for the circuit is indicated in Fig. 4, in a case where the value of E1 lstwice the value of E2 and where these values may, for example, be 20 and 10 volts respectively. The rectifiers II and 22 in this case may be, for example, an RCA-84 type tube; the resistor 53 will have a value of 680,000 ohms; the resistor 25, a value of 56,000 ohms; resistor 30, 56,000

ohms; and the capacitors I2 and 25 and the optional capacitor 3I each a value of .03 mi. These values give appropriate response characteristics for use in the circuit with a commercial type of RCA photophone variable area sound recording apparatus but, as indicated above, they should be changed to correspond with the characteristics of the apparatus with which the circuit is to be used. From these values, the values to be used in any-oi the other circuits may readily be approximately determined, although the exact values are at best determined empirically, due to the variation in characteristics of ground noise isduction apparatus even of the same type and variation in characteristics of tubes, resistors and capacitors.

I claim as my invention:

1. In combination, an input circuit, a timing circuit connected to said input circuit and having predetermined characteristics, a second timing circuit connected to said input circuit and having different characteristics, a common load circuit and means for combining the output potentials of said timing circuits in opposition to obtain a useful potential equal to the difierence between said output potentials for supplying said common load circuit.

2. In combination, an A.-C. input circuit, a rectifying and filtering circuit connected to said input circuit and having predetermined characteristics, a second rectifying and filtering circuit connected to said input circuit and having different characteristics, means for combining the output potentials of said rectifying and filtering circuits in opposition to obtain a potential equal to the difference between said output potentials, and a load circuit connected to said combining means for utilizing aid difference potential.

3. In combination, an A.-C. input circuit, a rectifying and filtering circuit connected to said input circuit and having predetermined time characteristics, a second rectifying and filtering circuit connected to receive a lower voltage from said input circuit and having more rapid characteristics, means for combining the output potentials of said rectifying and filtering circuits in opposition to obtain a, potential equal to the difference between said output potentials, and a load circuit connected to said combining means.

4. In combination, an A.-C. input circuit, a load circuit, rectifying means connected to said input circuit, filtering means having a predetermined time constant connected to said rectifying means and input circuit, a secondrectifying means and a second filtering means connected to said input circuit, said second filtering means having a shorter time constant than said first filtering means and being so connected to said input circuit as to have a lower voltage applied thereto, and means connecting the outputs of said filtering means in opposition to said load circuit.

- 5.'In combination, an A.-C. input circuit, a D.-C. output circuit, rectifying means connected to said input circuit, filtering means having a predetermined time constant connected to said rectifying means and input circuit, a second rectifying means and a second filtering means connected to said input circuit, said second filtering means having a shorter time constant than said first filtering means and being so connected to said input circuit as to have a lower voltage applied thereto, means connecting the potentials developed across said filtering means in opposition to said output circuit, and means for delaying increase in output potential of said second circuit on increase in input.

HILLEL I. REISKIND. 

